Method for the manufacture of a piezoelectric transducer and transducer manufactured by this method

ABSTRACT

Method and apparatus for the manufacture of piezoelectric transducers in which a tube spring is prestressed by an axial load with the tube welded to a transducer housing member.

United States Patent 91 Zieringer {451 May 22, 1973 [54] METHOD FOR THEMANUFACTURE OF A PIEZOELECTRIC TRANSDUCER AND TRANSDUCER MANUFACTURED BYTHIS METHOD [75] Inventor:

[73] Assignee: Hans List, Graz, Austria [22] Filed: Jan. 25, 1971 [21]Appl. No.: 109,443

Rudolf Zieringer, Graz, Austria Related U.S. Application Data [62]Division of Ser. No. 778,936, Nov. 26, 1968, abancloned.

[52] U.S. Cl. ..29/25.35, 29/497.5, 29/593,

310/87 [51] Int. Cl ..B01j 17/00, H04r 17/00 [58] Field of Search..29/25.35, 497.5,

[56] References Cited UNITED STATES PATENTS 2,486,482 11/1949 LaBrie..310/9.2 X 2,453,435 11/1948 Havstad ..310/9.2 X 3,621,547 11/1971Durham ..29/25.35

Primary Examiner-Charles W. Lanham Assistant Examiner-Carl E. HallAttorneyWatson, Cole, Grindle & Watson [57] ABSTRACT Method andapparatus for the manufacture of piezoelectric transducers in which atube spring is prestressed by an axial load with the tube welded to atransducer housing member.

3 Claims, 9 Drawing Figures PATENTEDMAY22 ma 3. 733,661

SHEET 3 [IF 3 METHOD FOR THE MANUFACTURE OF A PIEZOELECTRIC TRANSDUCERAND TRANSDUCER MANUFACTURED BY THIS METHOD This is a division ofapplication Ser. No. 778,936, filed Nov. 26, 1968 now abandoned.

The invention relates to a process for the manufacture of apiezoelectric transducer with a piezoelectric measuring element insertedin a tube spring and held under axial prestress between the inner frontend of the bottom of the tube spring and a lower end face of thetransducer housing or of a transducer housing insert serving as a rigidsupport, the tube spring being advanced with its open end close to thetransducer housing or to the transducer housing insert until such timewhen the measuring element directly adjoins the said end face, whereuponthe extremity of the tube spring is welded to the transducer housing orto the transducer housing insert under an axial load directed againstthe end face, thereby producing an annular tightly closed welding seam,the axial load being set prior to the completion of the weldingoperation to a value corresponding to the prestress of the measuringelement, on the one hand, and to an apparatus for the performance ofthis method of manufacture and to transducers manufactured by thismethod on the other hand.

Various expedients have already been devised for the purpose ofproducing a certain predetermined prestress in connection withtransducers wherein the axial prestress of the measuring elementrequired for the determination of values included in the negativemeasuring range, of values that is, tending to relieve the measuringinstrument, is produced by means of a tube spring. However, theseconventional methods are beset with a number of drawbacks.

It is possible, for example, in connection with conventional types oftransducers comprising a tube spring screwed to the transducer housing,to set the required prestress with a high degree of precision byappropriately screwing the tube spring. However, the provision of ascrew-coupling'between the tube spring and the transducer housing failsto assure hermetic sealing of the interior of the transducer containingthe measuring element against the outside as is indispensable for acontinuously smooth operation of the instrument. Furthermore, theoriginally set prestress is liable to alterations in the long run due toslackening of the screwcoupling.

According to another known method, the tube spring collar is pressedinto a housing and secured therein by flanging. In that case, therequired prestress can be set to an approximate value only with thenecessity of observing very narrow tolerances in length with regard tothe structural elements to be joined together. This design too, lacksthe necessary hermetic sealing.

Another method used for the manufacture of transducers consists insetting the measuring element clamped between the bottom of the tubespring and the support to the required prestress and in securing thetube spring by spotwelding at several points of its periphery to thetransducer housing insert defining the supporting surface. However,apart from the lack of hermetic sealing of the coupling, this method isrelatively unsafe since during the cooling of the welds, thermalstresses are likely to alter the prestress setting.

Better results are achievable by means of the aforesaid process whereinthe provision of a closed annular welding seam ensures the requiredhermetic sealing of the interior of the transducer. However, this knownmethod also involves a number of insecurity factors, particularly withregard to the obtention of an accurately predetermined prestress of themeasuring ele' ment, due to the fact that the measuring element isprestressed only in the course of the welding operation as a result ofthe progressive approach of the transducer elements to be weldedtogether and subject to a relative axial stress. Consequently, even ifthe axial stress acting upon the transducer elements clamped between thehollow electrodes of a resistance-welding apparatus is accuratelycontrolled, the prestress produced in the measuring elements uponcompletion of the welding operation may occasionally differ, howeverslightly, from the predetermined theoretical value. The amount ofprestress is furthermore influenced by other factors ascribable todifferences in the dimensions of the structural elements to be weldedtogether and also to fluctuations of amperage and of the duration ofvoltage application as well as to variations of the mechanical weldingpressures applied.

It is the object of the present invention to avoid the aforesaidshortcomings of known methods of manufacture and to provide a method forthe manufacture of piezoelectric transducers suitable for serialproduction, ensuring both the accurate observation of a predeterminedamount of prestress and the absolutely hermetic sealing of the interiorof the transducer. In conjunction with the known manufacturing processmentioned above, according to the invention the tube spring istherefore, advanced towards the transducer housing or transducer housinginsert and then subject to the full predetermined prestress by means ofan axial load, whereupon the tube spring is connected with thetransducer housing or transducer housing insert by arc welding,particularly by plasma welding, while the full prestress is maintainedall the time.

An essential advantage is derived from this process insofar as thewelding operation proper will practically not interfere with thepredetermined prestress which is maintained in a simple manner until theend of the welding operation. In particular, the arc welding method isresponsible for a local rise in temperature of the transducer componentsto be welded together which is, however, kept within narrow limits, sothat stress variations due to thermal expansion are practicallycompletely avoided.

As compared with conventional methods, the process according to theinvention offers the additional advantage that it is not absolutelynecessary for the manufacture of the tube spring and of the finishing ofthe corresponding counter-surface of the transducer housing or of thetransducer housing insert to strictly adhere to narrow tolerances in thelength of these structural elements.

Another outstanding feature of the present invention resides in the useof the plasma-welding process which is known to produce an almostpin-point concentration of an arc of extraordinary stability andfurthermore permitting the application of extremely low amperages.

According to a further embodiment of the invention the prestress of themeasuring element is preferably continuously measured in conjunctionwith the same in a manner known per se and automatically adjusted as afunction of the measured value so as to obtain the predeterminedtheoretical value. As a result of the automation of this operation whichis essential for the production of the required prestress, anydeviations of the final prestress of the measuring element from thepredetermined value due to operating errors or other individualinfluences are avoided. Besides, the manufacturing process is simplifiedand shortened.

According to a preferred embodiment of the invention the apparatus bywhich the latter is to be performed comprises two holders arrangedopposite and in coaxial relation to each other and displaceable inrelation to each other and to be loaded by opposing stresses, the saidholders serving for the tube spring containing the measuring element andthe transducer member comprising the supporting surface, as well as anelectric welding apparatus comprising an arc-type, particularly aplasma-type welding burner. By means of the apparatus it is possible tomechanize the manufacture of a transducer by the method according to theinvention and even to provide for an automatic manufacturing process bymeans of an appropriate program device.

Another preferred embodiment of the invention provides for the rotatablearrangement of one of the two holders on a fixed support, whereas theother holder is rotatably arranged on an axially displaceable abutment,preferably with the interposition of a dynamometer. In that case, it ispossible for the welding burner to be secured to a mount permitting onlythe conveyance of the burner to, and its removal from, the workpiece. Inthis connection, it is advisable according to another feature of theinvention, to provide at least one of the two holders with a rotarydrive tuned to the admissible advance speed of the welding burner. Thisrotary drive can be included in a simple manner in a program control forthe purpose of controlling the ignition of the arc of the welding burnerat the beginning of the continuous rotation of the workpiece and theswitchoff of the welding burner as soon as the workpiece has completed afull revolution.

In a piezoelectric transducer produced by the method according to theinvention and provided with a piezoelectric measuring element insertedin a tube spring and held under axial prestress between the inner frontend of the bottom of the tube spring and a lower end surface of thetransducer housing or of the transducer housing insert serving as arigid support, the open end of the prestressing sleeve may encompasspart of the transducer housing or of the transducer housing insert andform a gap with same at their junction, the said gap being bridged bythe annular welding seam. This gap ensures the formation of adeep-reaching welding seam since it facilitates the penetration of theburner jet. In order to take full advantage of the opportunities offeredby the method according to the invention, it is advisable to provide forsmall cross-sectional areas for the connection of the structuralelements to be welded together in the area of the gap formed by same,either by the provision of characteristic welding flanges on thesetransducer components or by having the crosssectional areas of samediminish in the direction of the gap.

According to another embodiment of the invention the transducer housingand/or the transducer housing insert and the open end of the tube springeach have an annular extension protruding in the direction of the sameside of the transducer, one extension surrounding the other and thewelding seam extending in the area of the adjacent front edges of thetwo extensions. The provision of characteristic annular extensions onthe transducer components to be interconnected makes it possible for thecross-sectional areas in these places which are directly exposed to theburner jet to be kept as small as possible in order to check heatconduction towards the remaining components of the transducer, therebyaccelerating the welding operation.

In line with this trend, the cross-sectional area of the extensions ispreferably so designed as to diminish in the direction of their freefront end.

According to a particularly simple design of the transducer according tothe present invention which is particularly suitable for purposes ofserial production, the extension having the smaller diameter is definedby an annular groove recessed in the associated transducer member.

Further details of the invention will appear from the followingdescription of the several embodiments of the invention with referenceto the accompanying drawing in which FIG. 1 is a schematic sectionalview showing the basic structure of an apparatus for the performance ofthe method according to the invention,

FIG. 2 is a vertical sectional view of a piezoelectric pressuretransducer manufactured in accordance with the method according to theinvention, prior to the welding operation,

FIG. 2a a detail of the pressure transducer as shown in FIG. 2 after thewelding operation,

FIG. 3 a variant of a transducer manufactured in accordance with thesame method, prior to the welding operation,

FIG. 3a a detail of the same transducer after the welding operation,

FIG. 4 an axial cross-sectional view of a piezoelectric accelerometerproduced in accordance with the method according to the invention priorto the welding operation, and

FIG. 4a a detail of this apparatus after the welding operation,

FIGS. 5a and 5b two consecutive stages of the process for themanufacture of a piezoelectric pressure transducer mounted in accordancewith the method according to the invention.

The apparatus shown in FIG. 1 comprises two holders l and 2 in oppositecoaxial relation to each other and stepped-down cylindrical in shape,one holder 1 containing in a blind bore 3 the tube spring 4 of the,piezoelectric transducer to be manufactured with the measuring element(not shown) inserted therein. The tube spring 4 rests with an annularshoulder defined by its reinforced inner rim 5 upon the annular frontend 6 of the holder 1.

Inserted in a corresponding blind bore 7 of the second holder 2 is thetransducer member, in the embodiment shown a transducer housing insert 9comprising the rigid support 8 (to be seen in FIG. 2). The transducerhousing insert 9 rests with a flange 10 on the annular front end 11 ofthe holder 2.

The holder 1 with its cylindrical shaft 12 is rotatably arranged in afixed support 13 and rests with a collar 14 axially upon the inner frontend of this fixed support 13.

The holder 2 is likewise rotatable and for that purpose inserted withits stepped-down cylindrical shaft in a bearing .16 of easy actionlocated at one end of a dynamometer 17 arranged in coaxial relation tothe two holders 1 and 2, the other end of the said dynamometer restingon an axially adjustable abutment 18. Obviously, it is possible for themechanical dynamometer 17 shown to be replaced by a load measuring cellor the like.

The apparatus illustrated in FIG. 1 further comprises an electricwelding burner which in this case is designed as a plasma-type weldingburner 19, the longitudinal axis of which is perpendicular to the axisof rotation of the two holders 1 and 2. The welding burner 19 isadjustable in an axial direction in the sense of the double arrow shown.The burner jet formed by a necked-down electric arc produced in a highlyionized gaseous envelope is directed to the very junction between thetube spring 4 and the transducer insert 9. The annular welding seamproduced by the burner 19 at the junction is designated by referencenumber 21.

Referring to FIGS. 2 and 2a illustrating a piezoelectric transducermanufactured in accordance with the method according to the invention'intwo different production stages before the welding operation (FIG. 2)and upon completion of the welding operation (FIG. 2a) the processaccording to the invention will hereafter be described as follows:

At the beginning, the measuring element 22 provided with a centralconnecting line 24 for the abduction of the charges produced is insertedin the tube spring 4 together with the surrounding insulating sleeve 23so as to rest with its inner front end on the bottom of the tube spring4. Now the tube spring 4 is inserted in the blind bore of holder 1. Theholder 2 receives the transducer housing insert 9 provided with centrallongitudinal bore comprising an insulating tube 26.

The tube spring 4 and the transducer housing insert 9 are now broughtclose to each other by appropriately shifting the abutment 18 until suchtime when the outer front end of the measuring element 22 comes intocontact with the support 8 defined by the front end of the transducerhousing insert 9. As shown in the left half of FIG. 2, a gap 27 of awidth h is left at the junction between the tube spring 4 and thetransducer insert 9.

Now the apparatus is axially loaded by the additional displacement ofthe abutment 18, as a result of which a prestress is produced in themeasuring element 22 clamped between the bottom 25 of the tube springand the support 8. This prestress is immediately made to correspond withthe predetermined theoretical value in accordance with the reading ofthe dynamometer l7. Thereupon the welding burner 19 is brought withinignition distance from the gap 27 and the main arc ignited. As thewelding operation begins, the shaft 12 of the holder 1 and consequently,the workpiece together with the holder 2 are rotated continuously as thewelding operation proceeds. The burner jet penetrating as far as the gap27 produces a tight homogeneous annular weld 21, bridging the gap (cf.FIG. 2a). Due to the high concentration of the are 20, heating of theworkpiece is restricted to the area directly adjoining the welding seam.The preset prestress of the measuring element is fully maintained duringthe welding operation. It is also possible for this process to beautomatically controlled, using the reading of the dynamometer 17 as acontrol quantity. It is, however, also possible to use the chargeproduced -in the measuring element 22 under the set prestress in amanner known per se as a control quantity for the purpose of maintainingthe predetermined prestress at a constant level.

As soon as the annular welding seam 21 is closed following a completerevolution of the shaft 12, the welding burner 19 is switched off andmoved back to its original position. Now the abutment 18 is also broughtinto its original position, whereupon the welded transducer is removedfrom the mounts l and 2.

The transducer illustrated in FIG. 2 is manufactured by the same method.The tube spring 4' containing the measuring element 22' together withthe connecting line 24' and an insulating sleeve 23 differs from thetype shown in FIG. 2 insofar as its reinforced rim 5' has a conicalannular front face 28 defining in conjunction with a correspondingconical surface 29 of the transducer housing insert 9 a gap 27. Theannular welding seam 21' shown in FIG. 3a is then produced in the areaof this gap 27'. The apparatus used for the welding of this transducerdiffers from the type shown in FIG. 1 only insofar as the welding burner19 is placed in an oblique position pointing in the direction of the gap27 The accelerometer shown in FIG. 4 comprises a hollow cylindricalhousing 31 with a connecting flange 32 (only part of which is shown inthe drawing), through which the two connecting lines 33 of the measuringelement 35 resting on the inner front end 34 of the housing bottomemerge. The measuring element 35 can be designed for example, as aconventional assembly of superposed quartz disks with electrodecoatings.

The seismic mass 36 loading the measuring element 35 rests upon theinner front end 37 of the bottom 38 of a tube spring 39 surrounding themeasuring unit and engaging with its open reinforced end 40 in thereceiving bore of the transducer housing 31.

The housing 31 as well as the end 40 of the tube spring each have anupwardly protruding annular extension 41 and 42 respectively, whosecross-sectional area diminishes in the direction of the free front end.

For the assembly of the accelerometer the same is clamped between twopressure feet 43 and 44 (indicated by dot-and-dash lines) of anauxiliary device (not shown) and loaded by an axial force producing apredetermined amount of prestress in the measuring element. While thisload is maintained, the jet of a plasmatype welding burner 45 alsoindicated by dot-and-dash lines in the drawing is directed upon the freefront end of the extensions 41 and 42 in such a manner that it impingesupon the contact edge of the two extensions.

In order to produce a closed annular welding seam,

the burner 45 is moved about the longitudinal axis of 4 the transducer,or, where a stationary burner is used, the transducer is axially rotatedby means of the two pressure feet 43 and 44. Upon completion of thewelding seam 46 (FIG. 4a) and after releasing the clamping by the twopressure feet, the assembly of the accelerometer is tenninated.Following cleaning and finishing of the weld where necessary, theapparatus is ready for use.

FIGS. 5a and 5b illustrates a piezoelectric pressure transducer whereina housing insert 47 and the tube spring 50 containing the measuringelement 48 together with an insulating sleeve 49 constitute the twotransducer elements to be welded together, which upon completion of thewelding operation are jointly inserted in a transducer housing 51(indicated by dashlines).

The reinforced extremity 52 of the tube spring and the housing insert 47present annular extensions 53 and 54 directed towards the same side ofthe transducer and are of a similar design as in the embodiment of theinvention hereabove described and illustrated in and by FIG. 4.Likewise, the transducer is assembled by clamping the transducerelements to be welded together between the pressure feet 55 and 56(indicated by dash-lines) of an auxiliary device (FIG. a). For thatpurpose, the housing insert 47 is provided with a collar 57 in which thepressure foot 55 engages. The other pressure foot 56 rests upon theannular shoulder 58 defined by the reinforced extremity 52 of the tubespring 50. The method of prestressing and welding by means of the burner45 is similar to that described with reference to the aforesaidembodiments of the invention,

but for the finishing of the shells of the housing insert I 47 and ofthe extremity 52 of the tube spring following completion of the weldingoperation, whereby the collar 57 is removed by turning and a shoulder 59is produced at the extremity 52 of the tube spring serving as an axialabutment of the transducer housing 51 for the final assembly of thepressure transducer (FIG. 5b).

It is obviously possible for the method according to the invention to beperformed also with devices other than the apparatus hereabove describedand as shown in the drawings. For example, non-rotatable holders can beprovided for the clamping of the transducer elements to be weldedtogether, with the welding burner rotating about the common longitudinalaxis of these holders as required. The devices used for that purpose maybe either hand-operated or automatically operated by means of a programcontrol system.

I claim:

1. A method for the manufacture of a piezoelectric transducer having atransducer housing, a rigid support formed by the front end of thehousing, a cylindrical outer surface of the housing concentric withrespect to the longitudinal axis of the transducer, a tube spring havingan open end facing the support and a bottom at its opposite end, acylindrical counter-surface concentric with the longitudinal axis of thetransducer at the open end of the tube spring and registering with thecylindrical surface of the transducer housing, a piezoelectric measuringelement mounted between the support and the bottom of the tube springand axially prestressed therebetween, and an annular weld on the openend of the tube spring adjoining the cylindrical counter-surfacethereof, comprising the steps of:

l. joining the tube spring with the transducer housing member adjacentto said cylindrical surface and said cylindrical counter-surface so thatthe measuring element abuts against said support and against the tubespring bottom,

2. prestressing the measuring element with a measured axial load uponthe transducer housing and the tube spring, and welding the open tubespring end to the transducer housing member adjacent welding edges ofthese members to provide an annular closed weld contiguous to saidfirst-mentioned welding edges, said welding being performedsimultaneously with continuously measuring the prestressed load of saidmeasuring element by means of the measuring element and automaticallyregulating the axial load as a function of said measured prestress tomaintain said prestress equivalent to a predetermined theoreticalprestress during the welding process.

2. The method according to claim 1 wherein the welding process isperformed by arc welding.

3. A method according to claim 1 wherein the welding process isperformed by plasma arc jet welding.

1. A method for the manufacture of a piezoelectric transducer having atransducer housing, a rigid support formed by the front end of thehousing, a cylindrical outer surface of the housing concentric withrespect to the longitudinal axis of the transducer, a tube spring havingan open end facing the support and a bottom at its opposite end, acylindrical counter-surface concentric with the longitudinal axis of thetransducer at the open end of the tube spring and registering with thecylindrical surface of the transducer housing, a piezoelectric measuringelement mounted between the support and the bottom of the tube springand axially prestressed therebetween, and an annular weld on the openend of the tube spring adjoining the cylindrical counter-surfacethereof, comprising the steps of:
 1. joining the tube spring with thetransducer housing member adjacent to said cylindrical surface and saidcylindrical counter-surface so that the measuring element abuts againstsaid support and against the tube spring bottom,
 2. prestressing themeasuring element with a measured axial load upon the transducer housingand the tube spring, and
 3. welding the open tube spring end to thetransducer housing member adjacent welding edges of these members toprovide an annular closed weld contiguous to said first-mentionedwelding edges, said welding being performed simultaneously withcontinuously measuring the prestressed load of said measuring element bymeans of the measuring element and automatically regulating the axialload as a function of said measured prestress to maintain said prestressequivalent to a predetermined theoretical prestress during the weldingprocess.
 2. prestressing the measuring element with a measured axialload upon the transducer housing and the tube spring, and
 2. The methodaccording to claim 1 wherein the welding process is performed by arcwelding.
 3. A method according to claim 1 wherein the welding process isperformed by plasma arc jet welding.
 3. welding the open tube spring endto the transducer housing member adjacent welding edges of these membersto provide an annular closed weld contiguous to said first-mentionedwelding edges, said welding being performed simultaneously withcontinuously measuring the prestressed load of said measuring element bymeans of the measuring element and automatically regulating the axialload as a function of said measured prestress to maintain said prestressequivalent to a predetermined theoretical prestress during the weldingprocess.